1. INTRODUCTION

Two thousand years after Lucretius proclaimed that nothing can be
created from nothing, inflationary cosmology asserts that he was
wrong.

WHEN AN OBSCURE RUSSIAN METEOROLOGIST named Alexander Friedmann
proposed, in 1922, that the Universe might be expanding, Albert
Einstein was sure that he was wrong. Five years earlier Einstein had
published a static model of the Universe, and he was still convinced
that it was correct. In a rare but dramatic blunder, Einstein
bolstered his unfounded beliefs with an erroneous calculation, and
fired off a note to the Zeitschrift fur Physik claiming that
Friedmann's theory violated the conservation of energy. Eight months
later, however, after a visit from a colleague of Friedmann's,
Einstein admitted his mistake and published a retraction. The
equations of general relativity do, he conceded, allow for the
possibility of an expanding universe.

Today the Big Bang theory, which began with Friedmann's calculations
in 1922, has become the accepted view of cosmology. The expansion of
the Universe was first observed in the early 1920s by Vesto Melvin
Slipher, and in 1929 was codified by Edwin Hubble into what we now
know as "Hubble's Law": on average, each distant galaxy is receding
from us with a velocity that is proportional to its distance. In 1965
Arno Penzias and Robert Wilson detected a background of microwave
radiation arriving at Earth from all directions - the afterglow of the
primordial hot, dense fireball. Today we know, based on data from the
Cosmic Background Explorer (COBE) satellite (see Beam
Line, Vol. 23, No. 3, Fall/Winter 1993), that the spectrum of this
background radiation agrees with exquisite precision - to 1/30 of 1
percent - with the thermal spectrum expected for the glow of hot
matter in the early Universe. In addition, calculations of
nucleosynthesis in the early universe show that the Big Bang theory
can correctly account for the cosmic abundance of the light nuclear
isotopes: hydrogen, deuterium, helium-3, helium-4, and lithium-7.
(Heavier elements, we believe, were synthesized much later, in the
interior of stars, and were then explosively ejected into interstellar
space.)

Despite the striking successes of the Big Bang theory, there is good
reason to believe that the theory in its traditional form is
incomplete. Although it is called the "Big Bang theory," it is not
really the theory of a bang at all. It is only the theory of the
aftermath of a bang. It elegantly describes how the early
Universe expanded and cooled, and how matter clumped to form galaxies
and stars. But the theory says nothing about the underlying physics of
the primordial explosion. It gives not even a clue about what banged,
what caused it to bang, or what happened before it banged. The
inflationary universe theory, on the other hand, is a description of
the bang itself, and provides plausible answers to these questions and
more.